Update phpunit constraints.

PHP 8.3 support is not required as CakePHP 6 is 8.4+

Author: ADmad

composer.json

@@ -31,7 +31,7 @@
     "require-dev": {
         "cakephp/cakephp-codesniffer": "^6.0",
         "cakephp/debug_kit": "^6.0",
-        "phpunit/phpunit": "^12.2.4 || ^13.0"
+        "phpunit/phpunit": "^13.0"
     },
     "autoload": {
         "psr-4": {